The potential of human embryonic (hESCs) and induced pluripotent stem cells (hiPSCs) to generate cardiovascular cells in culture provides a powerful model system for investigating cellular interactions and molecular regulators that govern the specification, commitment and maturation of these lineages, as well as a unique and unlimited source of human cardiomyocytes for drug testing and regenerative medicine strategies1-4. Translating this remarkable potential into practice is, however, dependent on technologies that enable the reproducible generation of highly enriched populations of cardiomyocytes, as contaminating cell types could impact drug responses and other functional properties in vitro and increase the risk for abnormal growth and teratoma formation following transplantation in vivo5. When induced under optimal cardiac conditions, human pluripotent stem cells (hPSCs) will efficiently differentiate to generate mixed cardiovascular populations, including cardiomyocytes, smooth muscle cells, fibroblasts and endothelial cells3. While cardiomyocytes can represent up to 70% of the population for any given hPSC line, the efficiency of generating this lineage does vary considerably between different stem cell lines. Further manipulation of induction conditions has not yet yielded strategies for the generation of pure populations of cardiomyocytes from a broad range of hPSC lines.
To enrich for cardiomyocytes from the differentiation cultures, cardiomyocyte-specific fluorescent reporters or drug selectable elements have been introduced into hPSCs6-8. Following differentiation, cardiomyocytes can be enriched either by fluorescent-activated cell sorting (FACS) or the addition of appropriate selection drugs. Although these strategies do allow for the generation of enriched cardiomyocyte populations, they suffer from a major drawback as a reporter vector must be introduced into each hPSC line used, resulting in genetically modified cardiomyocytes, thus reducing their utility for clinical applications. In a more recent study, Hattori et al. demonstrated that it was possible to isolate cardiomyocytes by FACS, based on their high mitochondrial content9. While this approach appears to be useful for isolating mature cardiomyocytes, cells with fewer mitochondria, such as immature hPSC-derived cardiomyocytes, may be more difficult to distinguish from other cell types.